The present invention relates to array antennas and more specifically to characterization of element patterns and amplifier characteristics in array antennas.
In an array antenna, an xe2x80x9cactive elementxe2x80x9d immersed in an array environment will behave differently from the case where the antenna element is removed from the array. This problem arises from mutual coupling between the antenna elements. Therefore if one is to have an accurate model for predicting its performance, the antenna element must be measured when the antenna element is placed in the array environment. In the prior art, the process is typically done by applying a source to the xe2x80x9cactive element,xe2x80x9d terminating the rest of the array elements, and then measuring the given active antenna element pattern.
Using the method of the prior art, single element pattern characterization measurements are used to determine each of the antenna element patterns. For an array of N elements, this is accomplished by exciting one array element and terminating all other Nxe2x88x921 array elements, such that only the desired array element is radiating energy. Only one of N array elements is measured at a time. Therefore, this is called the single antenna element approach. Using the single antenna element approach, all N antenna elements are measured sequentially. This process can be used to measure any array element pattern with the array element immersed in the array environment, which is, in general, different from an isolated array element, thus accounting for the mutual coupling interactions among array elements.
One problem with the prior art approach is that it is very time consuming since antenna elements are measured sequentially and the positioner will be required to go through the desired movement cycle once for each active array element. This is extremely inefficient and impractical when the positioner movement and data acquisition cycle must be repeated N times. A second disadvantage is that, in some cases, it may be difficult, impractical, or impossible to shut off all but one array element in the array under test. Removing signals from all but one array element may become a time consuming and expensive process, involving removal of a cable and replacement with a termination. If one is to rely on turning antenna elements off using digitally controlled radio frequency (RF) on/off switches, RF isolation may not be sufficient to allow for measurements to be performed to a suitable level of accuracy.
In a similar problem, the characterization of the amplitude and phase of each antenna element against signal level, frequency, and ambient temperature is crucial to create xe2x80x9clook-upxe2x80x9d calibration tables. This is particularly important in multi-beam active array antennas to characterize the nonlinear behavior of the amplifiers, and to compare them with theoretical models such as the Shimbo model; see O. Shimbo, xe2x80x9cTransmission Analysis in Communication Systems,xe2x80x9d Computer Science Press (1988). The current technique is to characterize each antenna element one at a time by disconnecting, turning off, or attenuating the other elements in the array. This is again the single antenna element approach so the technique is very time consuming, and therefore results in high parts integration and test time, which in turn adversely impacts the total assembly costs.
To further illustrate the limitations of the prior art, active phased-array antennas typically have a requirement to determine array element patterns while the antenna element is in the array environment. These data are needed for scaling factor constants which take into account that the antenna elements are at different distances from the calibration probe. The scaling factor constants are used in the near-field calibration system described in U.S. Pat. No. 6,084,545, issued Jul. 4, 2000 in the name of Lier et al. to take control circuit encoding (CCE) measurements of each of the array elements in an array antenna; see U.S. Pat. No. 5,572,219, issued Nov. 5, 1996 in the name of Silverstein et al. In other applications, accurate element patterns are needed for in-orbit far-field calibration where measurements of the main beam and sidelobes are taken for remote sensing of aperture deformation. For an array of 1000 elements, to efficiently obtain array element patterns for all the array elements, while the array elements are immersed in the array environment, 1000 cables must be disconnected and reconnected, the antenna rotated on a point, either spherical or planar, and the probe moved over the desired positioning range 1000 different times. This is a very time consuming and expensive process.
It can be understood then that the processes for measuring array element patterns and amplifier characteristics must be repeated for each of the array elements in the array antenna. The methods using the prior art are costly and inefficient since they are limited to measurements of a single array element at a time. Therefore, there is a need for performing antenna element pattern and amplifier characteristic measurements in a factory or diagnostic setting that allows all antenna elements and amplifiers to be characterized in an accurate, efficient and cost-effective way.
The system and method of the present invention described herein discloses a positioning device which allows movement of the antenna with respect to a calibration probe or movement of the calibration probe with respect to the antenna. It is the intermittent movement of the antenna and probe with respect to each other between measurement cycles which significantly improves implementation of the calibration procedure by permitting multiple simultaneous control circuit encoding (CCE) measurements of each of the array elements in an array antenna. The method is demonstrated experimentally using a near-field probe positioner to rapidly measure all 16 element patterns in a 2xc3x978 array of horns.
Similarly, the system and method of the present invention described herein discloses changes in the level of signals transmitted by the amplifiers in the elements of an array antenna system in conjunction with the use of orthogonal coding measurements. Changes in the level of signals transmitted significantly improves implementation of the process of determining amplifier characteristics by permitting simultaneous measurement of the amplifier characteristics of each of the array elements in an array antenna.
The present invention comprises a system for characterizing the patterns of a plurality of elements located in an array antenna, with each of the plurality of elements including at least one of (either or both) a phase shifter and an amplitude attenuator. The antenna includes a signal port for each individual beam which the array antenna generates, and a control signal input port to which control signals are applied for control of the phase shifters and amplitude attenuators. A plurality of antenna elements comprise a beamformer, a plurality of beamformers form the array antenna. The system for characterizing the patterns of a plurality of elements located in the array antenna system comprises: a probe positioned within the field of the array antenna, and positioning means for changing the relative position and orientation between the probe and the antenna. The system also includes a calibration radio-frequency source which is (a) coupled to at least one of the signal-ports of the array antenna when the array antenna is oriented as a transmit antenna, and (b) coupled to the probe when the array antenna is oriented as a receive antenna, with the calibration radio-frequency source generating a calibration signal. An orthogonal code generating means is applied to a plurality of antenna elements of at least one of the beamformers to sequentially set at least one of the phase shifters and amplitude attenuators (either one or both) with a plurality of sets of values. Each of the sets of values imposes a coding on the calibration signal to thereby sequentially generate calibration signals encoded with sets of values. Each set of values so encoded onto the calibration signals is orthogonal to other sets of values with which the calibration signals are encoded. When the array antenna is oriented as a transmit antenna, the probe receives the calibration signals sequentially encoded with mutually orthogonal values, and when the array antenna is oriented as a receive antenna, the calibration signals sequentially encoded with mutually orthogonal values are generated at least one of the signal ports of the array antenna. The system also includes a coherent radio-frequency receiver, a decoder for decoding signals encoded with the mutually orthogonal values, for generating decoded signals and means for coupling the encoded signals to the decoder, as a result of which the decoder generates the decoded signals. A processor is coupled to the decoder for processing the decoded signals for generating signals representing at least the values of one of phase shift and attenuation, or both if appropriate. The coupling means is coupled to the processor and to at least one of the phase shifters and the amplitude attenuators, for coupling to the signals representing at least the values of one of phase shift and attenuation. The system includes also a recorder for recording the signals representing at least the values of one set of probe-element positions and orientations and element characterization patterns; and an antenna controller for controlling the relative position between the probe and the antenna and for controlling the orthogonal code generating means. The probe is positioned typically at a position fixed relative to the array antenna. The position corresponds essentially to the boresight position of, and in front of, any one of the plurality of antenna elements by positioning means for changing the position of the probe relative to the array antenna by rotation of the array antenna around a vertical axis which is either coincident with the centerline of the probe, or parallel to the centerline of the probe. The present invention includes also a method of individually characterizing any or all of the antenna elements in an array antenna system simultaneously, without the need of performing sequential measurements, using the aforementioned system.
Another aspect of the present invention comprises a system for determining the characteristics of a plurality of amplifiers in an array antenna, with each amplifier coupled to an element located in the array antenna therein forming a plurality of elements, and each of the plurality of elements including at least one of (either one or both) a (a) phase shifter and (b) amplitude attenuator. The array antenna includes a beam port for each individual beam which the antenna generates, and a control signal input port to which control signals are applied for control of the phase shifters and amplitude attenuators. A plurality of antenna elements comprise a beamformer, and a plurality of beamformers forms the array antenna. The system for determining the characteristics of a plurality of amplifiers located i n the array antenna system comprises e a probe positioned at a distance fixed relative to the array antenna and within the field of the array antenna, and means for changing the strength level of signals applied to a plurality of amplifiers located in the array antenna. The system includes a calibration radio-frequency source, with the calibration radio-frequency source being (a) coupled to at least one of the signal ports of the array antenna when the array antenna is oriented as a transmit antenna, and (b) coupled to the probe when the array antenna is oriented as a receive antenna The system further comprises a calibration radio-frequency source generating a calibration signal; a calibration encoding means applied to a plurality of the antenna elements corresponding to any one of the beamformers for sequentially setting at least one of the phase shifters and the amplitude attenuators with a plurality of sets of values. Each of the sets of values imposes a coding on the calibration signal to thereby sequentially generate calibration signals encoded with sets of values. Each set of values so encoded onto the calibration signals is orthogonal to other sets of values with which the calibration signals are encoded, whereby, when the array antenna is oriented as a transmit antenna, the probe receives the calibration signals sequentially encoded with mutually orthogonal values, and when the array antenna is oriented as a receive antenna, the calibration signals sequentially encoded with mutually orthogonal values are generated at least one of the signal ports of the array antenna. The system includes also a coherent radio-frequency receiver; a decoder for decoding signals encoded with the mutually orthogonal values, for generating decoded signals therefrom; and encoded signal coupling means for coupling the encoded signals to the decoder, as a result of which the decoder generates the decoded signals. A processor is coupled to the decoder, for processing the decoded signals for generating signals representing at least the values of one of phase shift and attenuation, and coupling means are coupled to the processor and to at least one of the phase shifters and the amplitude attenuators, for processing the decoded signals for generating signals representing at least the values of at least one set of signal levels and amplifier characteristics. The system includes also a recorder for recording the signals representing at least the values of one set of probe element positions and orientations and amplifier characteristics; and an antenna controller for controlling the signal level changing means and the calibration encoding means. The probe is positioned at a position corresponding essentially to the boresight position of, and in front of, any one of the plurality of antenna elements. The amplifier system properties which can be determined include the output signal amplitude compared to the input signal amplitude and the relative phase between the output signal and the input signal. The present invention includes also method of individually characterizing any or all of the amplifier characteristics, such as output signal amplitude and phase versus input signal amplitude of amplifiers corresponding to a plurality of array elements in an array antenna system simultaneously, without the need of performing sequential measurements, using the aforementioned system.
In the method for characterizing the patterns of a plurality of elements located in an array antenna, each of the plurality of elements includes at least one of a (a) phase shifter and an (b) amplitude attenuator. The array antenna includes a signal port for each individual beam which the array antenna generates, and a control signal input port to which control signals are applied for control of the phase shifters and amplitude attenuators. A plurality of elements therein comprises a beamformer, and a plurality of beamformers forms the array antenna. The method for characterizing the patterns of a plurality of elements located in the array antenna comprises the steps of: positioning a probe within the field of the array antenna, with the probe and the array antenna fixed in position relative to each other; generating a calibration signal by means of a calibration radio-frequency source, the calibration radio-frequency source being (a) coupled to at least one of the signal-ports of the array antenna when the array antenna is oriented as a transmit antenna, and (b) coupled to the probe when the array antenna is oriented as a receive antenna; applying an orthogonal code generating means to a plurality of the antenna elements of the array antenna for sequentially setting at least one of the (a)phase shifters and (b)amplitude attenuators with a plurality of sets of values, each of the sets of values imposing a coding on the calibration signal to thereby sequentially generate calibration signals encoded with sets of values, each set of values so encoded onto the calibration signals being orthogonal to other sets of values with which the calibration signals are encoded, whereby, when the array antenna is oriented as a transmit antenna, the probe receives the calibration signals sequentially encoded with mutually orthogonal values, and when the array antenna is oriented as a receive antenna, the calibration signals sequentially encoded with mutually orthogonal values are generated at least one of the signal ports of the array antenna; receiving the calibration signals sequentially encoded with mutually orthogonal values by means of a coherent radio-frequency receiver; decoding signals encoded with the mutually orthogonal values by means of a decoder for generating decoded signals therefrom; coupling the encoded signals to the decoder as a result of which the decoder generates the decoded signals; processing by means of a processor coupled to the decoder the decoded signals for generating signals representing at least the values of one of phase shift and attenuation; coupling to the signals representing at least the values of one of phase shift and attenuation by coupling means coupled to the processor and to at least one of the phase shifters and the amplitude attenuators; recording the signals representing at least the values of one set of probe-element positions and element characterization patterns by means of a recorder; and controlling by means of an antenna controller the relative position between the probe and the antenna, and the orthogonal code generating means. The method includes using orthogonal coding to perform simultaneous measurements comprising at least one of (a) phase angles, and (b) amplitude levels of the phase shifters and attenuators of the plurality of elements corresponding to the array antenna; changing the relative position between the probe and the array antenna to a plurality of positions; using orthogonal coding to perform simultaneous recorded measurements comprising at least one of (a) phase angles and (b) amplitude levels of the plurality of elements at each of the plurality of positions in the array antenna; scaling the measurements of the relative probe element-probe positions by compensating for the pattern inherent to the probe, and recovering element patterns versus element-probe positions to characterize the patterns of elements of the array antenna.
In the method for determining the characteristics of a plurality of amplifiers in an array antenna, each amplifier is coupled to an element located in the array antenna therein forming a plurality of elements. Each of the plurality of elements includes at least one of a (a) phase shifter and an (b) amplitude attenuator, in which the array antenna includes a beam port for each individual beam which said antenna generates, and a control signal input port to which control signals are applied for control of said phase shifters and amplitude attenuators. A plurality of elements comprises a beamformer and a plurality of beamformers forms the array antenna. The method for determining the characteristics of a plurality of amplifiers located in the array antenna comprises the steps of: positioning a probe at a distance fixed relative to the array antenna, with the probe being within the field of the array antenna; applying a signal to a plurality of amplifiers located in any one of the beamformers of the array antenna; generating a calibration signal by means of a calibration radio-frequency source, the calibration radio-frequency source being (a) coupled to at least one of the signal ports of the array antenna when the array antenna is oriented as a transmit antenna, and (b) coupled to said probe when the array antenna is oriented as a receive antenna; applying an orthogonal code generating means to a plurality of the antenna elements corresponding to any one of the beamformers for sequentially setting at least one of the (a) phase shifters and (b) amplitude attenuators with a plurality of sets of values, each of the sets of values imposing a coding on the calibration signal to thereby sequentially generate calibration signals encoded with sets of values, each set of values so encoded onto said calibration signals being orthogonal to other sets of values with which the calibration signals are encoded, whereby, when the array antenna is oriented as a transmit antenna, the probe receives the calibration signals sequentially encoded with mutually orthogonal values and when the array antenna is oriented as a receive antenna, the calibration signals sequentially encoded with mutually orthogonal values are generated at least one of the signal ports of the array antenna; and receiving the calibration signals sequentially encoded with mutually orthogonal values by means of a coherent radio-frequency receiver. The method includes decoding by means of a decoder signals encoded with the mutually orthogonal values, for generating decoded signals therefrom; coupling by encoded signal coupling means the encoded signals to the decoder, as a result of which the decoder generates the decoded signals; processing by means of a processor coupled to the decoder the decoded signals for generating signals representing at least the values of one of phase shift and attenuation; coupling to the processor and to at least one of the phase shifters and the amplitude attenuators, for processing the decoded signals for generating signals representing at least the values of at least one set of signal levels and amplifier characteristics; recording by means of a recorder the signals representing at least the values of one set of probe element positions and orientations and amplifier characteristics; and controlling by means of an antenna controller the signal level changing means and the orthogonal code generating means. The method includes, in the case of a transmit antenna, setting the strength of the encoding signal to a plurality of signal input ports of the array antenna; in the case of a receive antenna, setting the strength of the encoding signal to the probe; using the orthogonal coding to perform simultaneous measurements comprising at least one of phase shift and attenuation of each amplifier corresponding to the plurality of elements in the array antenna; changing the signal strength levels to a plurality of signal strength levels; inserting recorded measurements of the input signal levels into the processor; and recovering recorded output signal levels versus input signal levels from the processor to determine the characteristics of a plurality of amplifiers in an array antenna.